The present invention generally relates to methods, materials and devices for improved electromagnetic/radiofrequency interference (EMI/RFI) shielding and thermal management in packaging electronic circuits. More particularly, the present invention relates to methods, materials and devices for the adjustment of viscosity of a thermally and/or electrically conductive, form-in-place, fully cured compound, utilized for EMI/RFI shielding and thermal management in packaging electronic circuits, thereby rendering the compound dispensable.
Circuit designs for modem electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like have become increasingly complex and compact. For example, integrated circuits have been manufactured for these and other devices which contain the equivalent of hundreds of thousands of transistors. Although the complexity of the designs has increased, the size of the devices has continued to shrink with improvements in the ability to manufacture smaller electronic components, and to pack more of these components in an ever smaller area.
As electronic components have become smaller and more densely packed on integrated boards and chips, designers and manufacturers now are faced with the challenge of how to dissipate the heat which is inevitably generated by these components. Indeed, it is well known that many electronic components, and especially power semiconductor components such as transistors and microprocessors, are more prone to failure or malfunction at high temperatures. Thus, the ability to dissipate heat often is a limiting factor on the performance of the component.
Electronic components within integrated circuits traditionally have been cooled via forced or convective circulation of air within the housing of the device. In this regard, cooling fins have been provided as an integral part of the component package or as separately attached thereto for increasing the surface area of the package exposed to convectively-developed air currents. Electric fans additionally have been employed to increase the volume of air which is circulated within the housing. For high power circuits and the smaller but more densely packed circuits typical of current electronic designs, however, simple air circulation often has been found to be insufficient to adequately cool the circuit components.
Heat dissipation beyond that which is attainable by simple air circulation may be effected by the direct mounting of the electronic component to a thermal dissipation member such as a “cold plate” or other heat sink or spreader. The dissipation member may be a dedicated, thermally-conductive ceramic or metal plate or finned structure, or simply the chassis or circuit board of the device. However, beyond the normal temperature gradients between the electronic component and the dissipation member, an appreciable temperature gradient is developed as a thermal interfacial impedance or contact resistance at the interface between the bodies.
The faying thermal interface surfaces of the component and heat sink typically are irregular, either on a gross or a microscopic scale. When the interface surfaces are mated, pockets or void spaces are developed therebetween in which air may become entrapped. These pockets reduce the overall surface area contact within the interface which, in turn, reduces the heat transfer area and the overall efficiency of the heat transfer through the interface. Moreover, as it is well known that air is a relatively poor thermal conductor, the presence of air pockets within the interface reduces the rate of thermal transfer through the interface.
To improve the heat transfer efficiency through the interface, a pad or other layer of a thermally-conductive, electrically-insulating material often is interposed between the heat sink and electronic component to fill in any surface irregularities and eliminate air pockets. Initially employed for this purpose were materials such as silicone grease or wax filled with a thermally conductive filler such as aluminum oxide. Such materials usually are semi-liquid or solid at normal room temperature, but may liquefy or soften at elevated temperatures to flow and better conform to the irregularities of the interface surfaces. Examples of such materials are described in U.S. Pat. Nos. 5,250,209; 5,167,851; 4,764,845; 4,473,113; 4,466,483; and 4,299,715, the pertinent disclosures of which are incorporated herein by reference in their entireties.
The greases and waxes of the aforementioned types heretofore known in the art, however, generally are not self-supporting or otherwise form-stable at room temperature, and are considered to be messy to apply to the interface surface of the heat sink or electronic component. To provide these materials in the form of a film which often is preferred for ease of handling, a substrate, web, or other carrier must be provided which introduces another interface layer in or between which additional air pockets may be formed. Moreover, the use of such materials typically involves hand application or lay-up by the electronics assembler which increases manufacturing costs.
Alternatively, another approach is to substitute a cured, sheet-like material in place of the silicone grease or wax. Such materials may be compounded and contain one or more thermally-conductive particulate fillers dispersed within a polymeric binder. These materials may be provided in the form of cured sheets, tapes, pads, or films. Typical binder materials include silicones, urethanes, thermoplastic rubbers, and other elastomers, with typical fillers including aluminum oxide, magnesium oxide, zinc oxide, boron nitride, and aluminum nitride.
Exemplary of the aforesaid interface materials are alumina or boron nitride-filled silicone or urethane elastomers which are marketed under the name CHO-THERM™ and THERM-A-GAP™ by the Parker-Hannifin Corporation. See, also, U.S. Pat. No. 4,869,954, which discloses a cured, form-stable, sheet-like, thermally-conductive material for transferring thermal energy. The material is formed of a urethane binder, a curing agent, and one or more thermally conductive fillers. The fillers, which may include aluminum oxide, aluminum nitride, boron nitride, magnesium oxide, or zinc oxide, range in particle size from about 1-50 microns (0.05-2 mils). Similar materials are described in U.S. Pat. Nos. 5,679,457; 5,545,473; 5,533,256; 5,510,174; 5,471,027; 5,359,768; 5,321,582; 5,309,320; 5,298,791; 5,213,868; 5,194,480; 5,151,777; 5,137,959; 5,060,114; 4,979,074; 4,974,119; 4,965,699; 4,869,954; 4,842,911; 4,782,893; 4,685,987; 4,654,754; 4,606,962; 4,602,678, and in WO 96/37915. Other materials, including gel or gel-like binders or carrier fillers, are described in U.S. Pat. Nos. 6,031,025; 5,929,138; 5,741,877; 5,665,809; 5,467,251; 5,079,300; 4,852,646; and in WO 96/05602, WO 00/63968; and EP 643,551. The respective disclosures of these references are incorporated herein by reference thereto.
Sheets, pads, and tapes of the above-described types have garnered general acceptance for use as interface materials in the conductive cooling of electronic component assemblies such as the semiconductor chips, i.e., dies, described in U.S. Pat. No. 5,359,768. In certain applications, however, heavy fastening elements such as springs, clamps, and the like are required to apply enough force to conform these materials to the interface surfaces in order to attain enough surface for efficient thermal transfer. Indeed, for some applications, materials such as greases and waxes which liquefy, melt, or soften at elevated temperature continue to be preferred as better able to conform to the interface surfaces under relatively low clamping pressures.
Recently, phase-change materials have been introduced which are self-supporting and form-stable at room temperature for ease of handling, but which liquefy or otherwise soften at temperatures within the operating temperature range of the electronic component to form a viscous, thixotropic phase which better conforms to the interface surfaces. These phase-change materials, which may be supplied as free-standing films, or as heated screens printed onto a substrate surface, advantageously function much like greases and waxes in conformably flowing within the operating temperature of the component under relatively low clamping pressures of about 5 psi (35 kPa). Such materials are further described in commonly-assigned U.S. Pat. No. 6,054,198 and U.S. application Ser. No. 09/212,111, filed Dec. 15, 1998 and entitled “Method of Applying a Phase Change Interface Material,” and are marketed commercially under the names THERMFLOW™ T310, T443, T705, T710, T725, and A725 by the Parker-Hannifin Corporation. Other phase-change materials are marketed commercially by the Bergquist Company (Minneapolis, Minn.) under the tradename HI-FLOW™, by Thermagon, Inc. (Cleveland, Ohio) under the tradename T-PCM™, and by Orcus, Inc. (Stilwell, Kans.) under the tradename THERMAPHASE™. A phase-change material/metal foil laminate is also marketed by Thermagon, Inc. under the tradename T-MATE™.
For a typical commercial application, the thermal interface material may be supplied in the form of a tape or sheet which includes an inner and outer release liner and an interlayer of a thermal compound. Unless the thermal compound is inherently tacky, one side of the compound layer may be coated with a thin layer of a pressure-sensitive adhesive (PSA) for the application of the compound to the heat transfer surface of a heat sink. In order to facilitate automated dispensing and application, the outer release liner and compound interlayer of the tape or sheet may be die cut to form a series of individual, pre-sized pads. Each pad thus may be removed from the inner release liner and bonded to the heat sink using the adhesive layer in a conventional “peel and stick” application which may be performed by the heat sink manufacturer.
With the pad being adhered to the heat transfer surface of the thermal dissipation member, such as a heat sink or spreader, and with the outer liner in place to form a protective cover, the outer surface of the compound layer, the dissipation member and the pad may be provided as an integrated assembly. Prior to installation of the assembly, the outer release liner is removed from the compound layer, and the pad positioned on the electronic component. A clamp may be used to secure the assembly in place.
Other materials are exemplified in U.S. Pat. No. 5,467,251, and in commonly-assigned U.S. Pat. No. 5,781,412. Materials marketed commercially by the Parker-Hannifin Corporation under the name THERM-A-FORM™ are commonly referred to as thermal interface compounds, caulks, form-in-place materials, or encapsulants. These materials typically are supplied as charged within one or more tubes, containers, and the like as, most often, one or two-part liquid or otherwise fluent, filled reactive systems which cure at room or elevated temperatures to form-in-place within the gap or component to which the compound is applied. Typical applicators include cartridge or tube guns or other dispensing systems.
In view of the variety of materials and applications available for use in thermal management, as exemplified in the foregoing, it is to be expected that continued improvements in such materials and applications in thermal management materials would be useful to electronics manufacturers.